In a typical vehicle air-conditioning system, refrigerant is compressed by a compressor unit driven by the automobile engine. The compressed refrigerant, at high temperature and pressure, enters a condenser where heat is removed from the compressed refrigerant. The refrigerant then travels through a receiver/dryer to an expansion valve. The expansion valve throttles the refrigerant as it flows through a valve orifice, which causes the refrigerant to change phase from liquid to a saturated liquid/vapor mixtures as it enters the evaporator. In the evaporator, heat is drawn from the environment to replace the latent heat of vaporization of the refrigerant, thus cooling the environmental air. The low pressure refrigerant flow from the evaporator returns to the suction side of the compressor to begin the cycle anew.
The high pressure refrigerant flow through the expansion valve must be regulated in response to the degree of superheat of the refrigerant flow between the evaporator and suction side of the compressor to maximize the performance of the air-conditioning system. The superheat is defined as the temperature difference between the actual temperature of the low pressure refrigerant flow and the temperature of evaporation of the flow.
Thermostatic expansion valves typically include a power element comprising a diaphragm mounted between a domed head and a support cup on the valve body. A "charge" is located within a head chamber defined by the domed head and one (upper) surface of the diaphragm. The support cup and the other (lower) surface of the diaphragm define a diaphragm chamber with the body of the expansion valve. A valve stem extends downwardly from the diaphragm through a bore in the valve body to a valve element modulating a valve orifice between a first port in the valve body (to the condenser) and a second port in the valve body (to the evaporator). Proctor, U.S. Pat. No. 3,667,247; Treder; U.S. Pat. No. 3,537,645; and Orth, U.S. Pat. No. 4,542,852, for example, show such expansion valves.
To control the refrigerant flow, the diaphragm in the power element moves in response to the refrigerant condition exiting the evaporator and compensates the flow rate to the evaporator by opening or closing the valve orifice. One type of device used to communicate the refrigerant condition to the diaphragm is a feeler bulb. The feeler bulb is positioned in contact with the pipe carrying the refrigerant, and a tube extends from the feeler bulb to the diaphragm chamber such that the refrigerant charge in the diaphragm chamber is at essentially the temperature of the refrigerant at the location of the bulb. Refrigerant pressure against the bottom of the diaphragm along with the force of an adjustment spring on the valve element tends to close the valve, while pressure from the charge tends to open the valve.
Another, more recent device to sense the degree of flow superheat is the block-type ("bulbless") thermostatic expansion valve. In certain bulbless valves, a thermally conductive pressure pad is located against the lower surface of the diaphragm. As the refrigerant passes around the pressure pad, heat energy is transferred by conduction through the pad to the refrigerant charge in the head chamber above the diaphragm valve. A portion of the diaphragm surrounding the pressure pad is typically also exposed and in direct contact with the refrigerant. The balance of forces across the diaphragm along with the spring constant of the diaphragm determine the deflection of the diaphragm and hence the opening of the expansion valve orifice between the condenser and evaporator. The diaphragm deflects as appropriate to maintain a balance between these forces.
A typical valve element for a thermostatic expansion valve includes a ball located in a ball retainer or ball seat which is biased by a spring against the valve orifice between the condenser port and the evaporator port. It is also known to support the ball directly against the end coil of the spring, and to use a cone-shaped element instead of a ball. In any case, the valve steam engages the ball and urges the ball away from the orifice in response to movement of the diaphragm. The spring is held in place by a gland or spring seat, which can be screwed into the passage leading to the outlet in the body. Adjustment of the axial location of the gland (such as by screwing the gland into or out of the valve body) adjusts the spring force on the valve ball, and hence adjusts the flow through the valve. The ball retainer and gland are typically formed from brass, and are otherwise separated from each other by the spring.
While the valve element described above performs satisfactorily for many applications, the various components of the element can be difficult to assemble and can fall apart and become lost when the element is removed, such as for inspection and repair. The components are also formed from a material (brass) that is relatively expensive and time-consuming to manufacture.
Thus, applicants believe that there is a demand for an improved valve element for thermostatic expansion valves which overcomes these shortcomings, and in particular for a valve element that is provided as an integral assembly where the different components of the valves element cannot become easily disassembled when removed from the valve, and which is formed from components that are relatively inexpensive to manufacture.